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Amazon River Reversed Flow

Ask any South American dinosaur which way the Amazon River flows and she would have told you east-to-west, the opposite of today. That’s the surprising conclusion of researchers studying ancient mineral grains buried in the Amazon Basin.

The once westward roll of what is now the world’s largest river was caused by a long-gone highland near what today is the river’s mouth. That highland was created by the breaking away of South America from Africa and the creation of the Atlantic Ocean during the Cretaceous Period, 65 to 145 million years ago. Later, when the Andes rose up on the western side of South America, the river had no choice but to drain into the new ocean.

“It just happened in a way that the current Amazon could take advantage of where an old river and ocean basin used to sit,” said geologist Russell Mapes, a doctoral candidate at the University of North Carolina in Chapel Hill.

Previous Brazilian and U.S. researchers have proposed smaller scale reversals and splits in the Amazon Basin, but nothing on the scale of the entire basin, said Mapes.

The evidence for the Amazon’s ancient switcheroo comes in the form of tiny crystals of a mineral called zircon, as well as telltale signs of the river flow direction captured in the structure of old river sediments.

Zircons are stubbornly long-lived and tend to be recycled over and over without stopping their internal uranium-lead radioisotope clocks that started ticking when the minerals first formed. As a result, they are tiny windows for peaking at long-lost mountains and entire continents.

The zircons in old sands studied by Mapes, his UNC faculty advisor Drew Coleman, and their Brazilian colleagues Afonso Nogueira and Angela Maria Leguizamon Vega, stand out because they do not appear to come from the Andes at all. In fact they date to about 1.3 to 2.1 billion years ago. So they had to have been formed in rocks that solidified in mountains that eroded away into the earliest Amazon.

Mapes is scheduled to present the team’s findings on Wednesday, 25 October, at the Annual Meeting of the Geological Society of America in Philadelphia. What he won’t have time to describe to other researchers is the adventure they underwent to gather up those little zircons.

“It was a pretty exotic trip,” said Coleman. “We took planes and aluminum small boats. In the first year we took a two-and-a-half week cruise up the river from Manaus. We sampled young and old sediment outcrops on the river. We traveled about 500 kilometers.”

“In the second year, we did three small hops -- plane flights downstream then rented small boats at each stopping point,” recalls Mapes. “One night in a boat we ran out of gas in an electrical storm. Luckily, it was the one place in the basin where my cell phone worked. We called the man who rented the boat to us and he came out and got us. We just bobbed around. It was a place where the river is several miles wide.”

In the end, said Mapes, it was worth it to finally be able to see back further in time. “When I got the actual data back, I was happy.”